Resistance projection welding is a non-polluting mechanized process used to obtain an assembly between similar or dissimilar metallic materials. The main advantages of this welding process are the possibility to achieve many different welded points at the same time and the long life of the electrodes compared to the spot-welding process. The paper analyses the effects of thin aluminium coating existing on mild steel parts of on the correct formation of welding points when assembling moulds for the manufacture of baking bread. From optical and electron microscopy analyses it resulted that some adjacent welded points show an interrupted fusion line, sprinkled with elongated islands of aluminium-rich compounds. The paper presents the effect of changing the values ​​of the welding parameters on the weld spot size, in correlation with the Al-rich inclusions that appear on the weld fusion zone. The best results have been obtained when the welding parameters values were the follows: electrode pressure of 2.6 bar, welding power of 19.18kVA and welding time of 7ms. The problems that occur when electric resistance welding of parts with aluminium coating have been highlighted, being useful for specialists who make products using this welding process.